peer: Track and return advertised protocol version
peer: Track and return advertised protocol version

This adds a new field to the peer struct which stores the protocol
version advertised by the remote peer and updates the StatsSnapshot to
return the advertised version instead of the negotiated version.

davecgh/btcd,ltcsuite/ltcd,ppcsuite/ppcd,Stash-Crypto/btcd,jongillham/btcd,cqswxj2013/btcd,btcsuite/btcd,adiabat/btcd,cqswxj2013/btcd,wallclockbuilder/btcd,Roasbeef/btcd,btcsuite/btcd,ppcsuite/ppcd,dajohi/btcd,wallclockbuilder/btcd,adiabat/btcd,davecgh/btcd,dajohi/btcd,Roasbeef/btcd,tb00/btcd,Stash-Crypto/btcd,dajohi/btcd,aakselrod/btcd,stevenroose/btcd,aakselrod/btcd,jongillham/btcd,tb00/btcd,stevenroose/btcd,ltcsuite/ltcd